If the person is loud and straight forward, you may think that person is rude.
If the person is quiet, you might think he/she is anti social .
And the list goes on...
You never really know, it might be that person's nature to be loud/quiet and so on. And because you never knew that in the first place, you tend to judge and assume. I won't deny that I've done that a few times myself, but hey everyone does that too.
Coming to friends, there's always something that your friends would love to do, it's a habit, and it annoys the shit out of you. For example if your friend curses at you as though it's a common word, what would you do? You'll probably flinch and that get angry afterwards. What if that's how your friend communicates, if you knew that from the start you would not react as bad as how you initially did isn't it?
Your friend could be too friendly at times, sometimes even perceived as being a flirt. But if it's in his or her nature, what can they do about it, other than not being themselves.
Anyway what I'm trying to say is that communication is so important, be it a normal conversation, friendships or even a relationship. IF they matter to you, know everything about them. I'm not saying that every bad thing about them should be tolerated, but its better than not knowing about it. Who knows, the smallest detail you missed could ruin things for you.